Fields serve as foundational notions in several mathematical domains. This includes different branches of analysis, which are based on fields with additional structure. Basic theorems in analysis hinge on the structural properties of the field of real numbers. Most importantly for algebraic purposes, any field may be used as the scalars for a vector space, which is the standard general context for linear algebra. Number fields, the siblings of the field of rational numbers, are studied in depth in number theory. Function fields can help describe properties of geometric objects.

Informally, a field is a set, along with two operations defined on that set: an addition operation written as a + b, and a multiplication operation written as a ⋅ b, both of which behave similarly as they behave for rational numbers and real numbers, including the existence of an additive inverse−a for all elements a, and of a multiplicative inverseb−1 for every nonzero element b. This allows us to consider also the so-called inverse operations of subtractiona − b, and divisiona / b, via defining:

Formally, a field is a setF together with two operations called addition and multiplication.[1] An operation is a mapping that associates an element of the set to every pair of its elements. The result of the addition of a and b is called the sum of a and b and denoted a + b. Similarly, the result of the multiplication of a and b is called the product of a and b, and denoted ab or a⋅b. These operations are required to satisfy the following properties, referred to as field axioms. In the following definitions, a, b and c are arbitrary elements of the field F.

Fields can also be defined in different, but equivalent ways. One can alternatively define a field by four binary operations (add, subtract, multiply, divide), and their required properties. Division by zero is, by definition, excluded.[2] In order to avoid existential quantifiers, fields can be defined by two binary operations (addition and multiplication), two unary operations (yielding the additive and multiplicative inverses, respectively), and two nullary operations (the constants 0 and 1). These operations are then subject to the conditions above. This approach avoids existential quantifiers, which is important in constructive mathematics and computing.[3]

Rational numbers have been widely used a long time before the elaboration of the concept of field. They are numbers which can be written as fractionsa/b, where a and b are integers, and b ≠ 0. The additive inverse of such a fraction is −a/b, and the multiplicative inverse (provided that a ≠ 0) is b/a, which can be seen as follows:

The multiplication of complex numbers can be visualized geometrically by rotations and scalings.

The real numbersR, with the usual operations of addition and multiplication, also form a field. The complex numbersC consist of expressions

a + bi

where i is the imaginary unit, i.e., a (non-real) number satisfying i2 = −1. Addition and multiplication of real numbers are defined in such a way that all field axioms hold for C. For example, the distributive law enforces

The complex numbers form a field. Complex numbers can be geometrically represented as points in the plane, and addition resp. multiplication of such numbers then corresponds to adding resp. rotating and scaling points. The fields of real and complex numbers are used throughout mathematics, physics, engineering, statistics, and many other scientific disciplines.

The geometric mean theorem asserts that h2 = pq. Choosing q = 1 allows construction of the square root of a given constructible number p. Construct the segments AB, BD, and a semicircle over AD (center at the midpointC), which intersects the perpendicular line through B in a point F, exactly h=p{\displaystyle h={\sqrt {p}}} apart from B.

In antiquity, several geometric problems concerned the (in)feasibility of constructing certain numbers with compass and straightedge. For example, it was unknown to the Greeks that it is in general impossible to trisect a given angle. These problems can be settled using the field of constructible numbers.[5] Real constructible numbers are, by definition, lengths of line segments that can be constructed from the points 0 and 1 in finitely many steps using only compass and straightedge. These numbers, endowed with the field operations of real numbers, restricted to the constructible numbers, form a field, which properly includes the field Q of rational numbers. The illustration shows the construction of square roots of constructible numbers, not necessarily contained within Q.

Not all real numbers are constructible. It can be shown that 23{\displaystyle {\sqrt[{3}]{2}}} is not a constructible number, which implies that it is impossible to construct with compass and straightedge the length of the side of a cube with volume 2, another problem posed by the ancient Greeks.

In addition to familiar number systems such as the rationals, there are other, less immediate examples of fields. The following example is a field consisting of four elements called O, I, A, and B. The notation is chosen such that O plays the role of the additive identity element (denoted 0 in the axioms above), and I is the multiplicative identity (denoted 1 in the axioms above). The field axioms can be verified by using some more field theory, or by direct computation. For example,

A · (B + A) = A · I = A, which equals A · B + A · A = I + B = A, as required by the distributivity.

This field is called a finite field with four elements, and is denoted F4 or GF(4).[6] The subset consisting of O and I (highlighted in red in the tables at the right) is also a field, known as the binary fieldF2 or GF(2). In the context of computer science and Boolean algebra, O and I are often denoted respectively by false and true, the addition is then denoted XOR (exclusive or), and the multiplication is denoted AND. In other words, the structure of the binary field is the basic structure that allows computing with bits.

The axioms of a field F imply that it is an abelian group under addition. This group is called the additive group of the field, and is sometimes denoted by (F, +) when denoting it simply as F could be confusing.

Similarly, the nonzero elements of F form an abelian group under multiplication, called the multiplicative group, and denoted by (F \ {0}, ·) or just F \ {0} or F*.

A field may thus be defined as set F equipped with two operations denoted as an addition and a multiplication such that F is a group under addition, F \ {0} is a group under multiplication (where 0 is the identity element of the addition), and multiplication is distributive over addition.[nb 1] Some elementary statements about fields can therefore be obtained by applying general facts of groups. For example, the additive and multiplicative inverses −a and a−1 are uniquely determined by a.

The requirement 1 ≠ 0 follows, because 1 is the identity element of a group that does not contain 0.[8] Thus, the trivial ring, consisting of a single element, is not a field.

In addition to the multiplication of two elements of F, it is possible to define the product n ⋅ a of an arbitrary element a of F by a positive integern to be the n-fold sum

a + a + ... + a (which is an element of F.)

If there is no positive integer such that

n ⋅ 1 = 0,

then F is said to have characteristic 0.[9] For example, Q has characteristic 0 since no positive integer n is zero. Otherwise, if there is a positive integer n satisfying this equation, the smallest such positive integer can be shown to be a prime number. It is usually denoted by p and the field is said to have characteristic p then. For example, the field F4 has characteristic 2 since (in the notation of the above addition table) I + I = O.

If F has characteristic p, then p ⋅ a = 0 for all a in F. This implies that

is compatible with the addition in F (and also with the multiplication), and is therefore a field homomorphism.[10] The existence of this homomorphism makes fields in characteristic p quite different from fields of characteristic 0.

A subfieldE of a field F is a subset of F that is a field with respect to the field operations of F. Equivalently E is a subset of F that contains 1, and is closed under addition, multiplication, additive inverse and multiplicative inverse of a nonzero element. This means that 1 ∊ E, that for all a, b ∊ E both a + b and a · b are in E, and that for all a ≠ 0 in E, both –a and 1/a are in E.

Field homomorphisms are maps f: E → F between two fields such that f(e1 + e2) = f(e1) + f(e2), f(e1e2) = f(e1)f(e2), and f(1E) = 1F, where e1 and e2 are arbitrary elements of E. All field homomorphisms are injective.[11] If f is also surjective, it is called an isomorphism (or the fields E and F are called isomorphic).

A field is called a prime field if it has no proper (i.e., strictly smaller) subfields. Any field F contains a prime field. If the characteristic of F is p (a prime number), the prime field is isomorphic to the finite field Fp introduced below. Otherwise the prime field is isomorphic to Q.[12]

Finite fields (also called Galois fields) are fields with finitely many elements, whose number is also referred to as the order of the field. The above introductory example F4 is a field with four elements. Its subfield F2 is the smallest field, because by definition a field has at least two distinct elements 1 ≠ 0.

In modular arithmetic modulo 12, 9 + 4 = 1 since 9 + 4 = 13 in Z, which divided by 12 leaves remainder 1. Since 12 is not a prime, Z/12Z is not a field, though.

The simplest finite fields, with prime order, are most directly accessible using modular arithmetic. For a fixed positive integer n, arithmetic "modulo n" means to work with the numbers

Z/nZ = {0, 1, ..., n − 1}.

The addition and multiplication on this set are done by performing the operation in question in the set Z of integers, dividing by n and taking the remainder as result. This construction yields a field precisely if n is a prime number. For example, taking the prime n = 2 results in the above-mentioned field F2. For n = 4 and more generally, for any composite number (i.e., any number n which can be expressed as a product n = r⋅s of two strictly smaller natural numbers), Z/nZ is not a field: the product of two non-zero elements is zero since r⋅s = 0 in Z/nZ, which, as was explained above, prevents Z/nZ from being a field. The field Z/pZ with p elements (p being prime) constructed in this way is usually denoted by Fp.

Every finite field F has q = pn elements, where p is prime and n ≥ 1. This statement holds since F may be viewed as a vector space over its prime field. The dimension of this vector space is necessarily finite, say n, which implies the asserted statement.[13]

A field with q = pn elements can be constructed as the splitting field of the polynomial

f(x) = xq − x.

Such a splitting field is an extension of Fp in which the polynomial f has q zeros. This means f has as many zeros as possible since the degree of f is q. For q = 22 = 4, it can be checked case by case using the above multiplication table that all four elements of F4 satisfy the equation x4 = x, so they are zeros of f. By contrast, in F2, f has only two zeros (namely 0 and 1), so f does not split into linear factors in this smaller field. Elaborating further on basic field-theoretic notions, it can be shown that two finite fields with the same order are isomorphic.[14] It is thus customary to speak of the finite field with q elements, denoted by Fq or GF(q).

for a prime p and, again using modern language, the resulting cyclic Galois group. Gauss deduced that a regular p-gon can be constructed if p = 22k + 1. Building on Lagrange's work, Paolo Ruffini claimed (1799) that quintic equations (polynomial equations of degree 5) can not be solved algebraically, however his arguments were flawed. These gaps were filled by Niels Henrik Abel in 1824.[18]Évariste Galois, in 1832, devised necessary and sufficient criteria for a polynomial equation to be algebraically solvable, thus establishing in effect what is known as Galois theory today. Both Abel and Galois worked with what is today called an algebraic number field, but conceived neither an explicit notion of a field, nor of a group.

In 1871 Richard Dedekind introduced, for a set of real or complex numbers which is closed under the four arithmetic operations, the German word Körper, which means "body" or "corpus" (to suggest an organically closed entity). The English term "field" was introduced by Moore (1893).[19]

By a field we will mean every infinite system of real or complex numbers so closed in itself and perfect that addition, subtraction, multiplication, and division of any two of these numbers again yields a number of the system.

In 1881 Leopold Kronecker defined what he called a "domain of rationality", which is a field of rational fractions in modern terms. Kronecker's notion did not cover the field of all algebraic numbers (which is a field in Dedekind's sense), but on the other hand was more abstract than Dedekind's in that it made no specific assumption on the nature of the elements of a field. Kronecker interpreted a field such as Q(π) abstractly as the rational function field Q(X). Prior to this, examples of transcendental numbers were known since Joseph Liouville's work in 1844, until Charles Hermite (1873) and Ferdinand von Lindemann (1882) proved the transcendence of e and π, respectively.[21]

A commutative ring is a set, equipped with an addition and multiplication operation, satisfying all the axioms of a field, except for the existence of multiplicative inverses a−1.[24] For example, the integers Z form a commutative ring, but not a field: the reciprocal of an integer n is not itself an integer, unless n = ±1.

In the hierarchy of algebraic structures fields can be characterized as those commutative rings R in which every nonzero element is a unit (which means every element is invertible). Similarly, fields are those commutative rings which have precisely two distinct ideals, (0) and R. Fields are also precisely those commutative rings in which (0) is the only prime ideal.

Given a commutative ring R, there are two ways to construct a field related to R, i.e., two ways of modifying R such that all nonzero elements become invertible: forming the field of fractions, and forming residue fields. The field of fractions of Z is Q, the rationals, while the residue fields of Z are the finite fields Fp.

Given an integral domainR, its field of fractionsQ(R) is built with the fractions of two elements of R exactly as Q is constructed from the integers. More precisely, the elements of Q(R) are the fractions a/b where a and b are in R, and b ≠ 0. Two fractions a/b and c/d are equal if and only if ad = bc. The operation on the fractions work exactly as for rational numbers. For example,

over a field F is the field of fractions of the ring F[[x]] of formal power series (in which k ≥ 0). Since any Laurent series is a fraction of a power series divided by a power of x (as opposed to an arbitrary power series), the representation of fractions is less important in this situation, though.

This field F contains an element x (namely the residue class of X) which satisfies the equation

f(x) = 0.

For example, C is obtained from R by adjoining the imaginary unit symbol i which satisfies f(i) = 0, where f(X) = X2 + 1. Moreover, f is irreducible over R, which implies that the map which sends a polynomial f(X) ∊ R[X] to f(i) yields an isomorphism

Fields can be constructed inside a given bigger container field. Suppose given a field E, and a field F containing E as a subfield. For any element x of F, there is a smallest subfield of F containing E and x, called the subfield of F generated by x and denoted E(x).[27] The passage from E to E(x) is referred to by adjoining an element to E. More generally, for a subset S ⊂ F, there is a minimal subfield of F containing E and S, denoted by E(S).

The compositum of two subfields E and E' of some field F is the smallest subfield of F containing both E and E'. The compositum can be used to construct the biggest subfield of F satisfying a certain property, for example the biggest subfield of F which is, in the language introduced below, algebraic over E.[nb 2]

for appropriate coefficientsen, ..., e0 ∈ E, en ≠ 0. For example, i ∊ C is algebraic over R and even over Q since it satisfies the equation

i2 + 1 = 0.

A field extension in which every element of F is algebraic over E is called an algebraic extension. Any finite extension is necessarily algebraic, as can be deduced from the above multiplicativity formula.[29]

The subfield E(x) generated by an element x, as above, is an algebraic extension of E if and only if x is an algebraic element. That is to say, if x is algebraic, all other elements of E(x) are necessarily algebraic as well. Moreover, the degree of the extension E(x) / E, i.e., the dimension of E(x) as an E-vector space, equals the minimal degree n such that there is a polynomial equation involving x, as above. If this degree is n, then the elements of E(x) have the form

For example, the field Q(i) of Gaussian rationals is the subfield of C consisting of all numbers of the form a + bi where both a and b are rational numbers: summands of the form i2 (and similarly for higher exponents) don't have to be considered here, since a + bi + ci2 can be simplified to a − c + bi.

The above-mentioned field of rational fractionsE(X), where X is an indeterminate, is not an algebraic extension of E since there is no polynomial equation with coefficients in E whose zero is X. Elements, such as X, which are not algebraic are called transcendental. Informally speaking, the indeterminate X and its powers do not interact with elements of E. A similar construction can be carried out with a set of indeterminates, instead of just one.

Once again, the field extension E(x) / E discussed above is a key example: if x is not algebraic (i.e., x is not a root of a polynomial with coefficients in E), then E(x) is isomorphic to E(X). This isomorphism is obtained by substituting x to X in rational fractions.

has a solution x ∊ F.[31] By the fundamental theorem of algebra, C is algebraically closed, i.e., any polynomial equation with complex coefficients has a complex solution. The rational and the real numbers are not algebraically closed since the equation

x2 + 1 = 0

does not have any rational or real solution. A field containing F is called an algebraic closure of F if it is algebraic over F (roughly speaking, not too big compared to F) and is algebraically closed (big enough to contain solutions of all polynomial equations).

Any field F has an algebraic closure, which is moreover unique up to (non-unique) isomorphism. It is commonly referred to as the algebraic closure and denoted F. For example, the algebraic closure Q of Q is called the field of algebraic numbers. The field F is usually rather implicit since its construction requires the ultrafilter lemma, a set-theoretic axiom which is weaker than the axiom of choice.[32] In this regard, the algebraic closure of Fq, is exceptionally simple. It is the union of the finite fields containing Fq (the ones of order qn). For any algebraically closed field F of characteristic 0, the algebraic closure of the field F((t)) of Laurent series is the field of Puiseux series, obtained by adjoining roots of t.[33]

A field F is called an ordered field if any two elements can be compared, so that x + y ≥ 0 and xy ≥ 0 whenever x ≥ 0 and y ≥ 0. For example, the reals form an ordered field, with the usual ordering ≥. The Artin-Schreier theorem states that a field can be ordered if and only if it is a formally real field, which means that any quadratic equation

An Archimedean field is an ordered field such that for each element there exists a finite expression

1 + 1 + ··· + 1

whose value is greater than that element, that is, there are no infinite elements. Equivalently, the field contains no infinitesimals (elements which are smaller than all rational numbers); or, yet equivalent, the field is isomorphic to a subfield of R.

Each bounded real set has a least upper bound.

An ordered field is Dedekind-complete if all upper bounds, lower bounds (see Dedekind cut) and limits, which should exist, do exist. More formally, each bounded subset of F is required to have a least upper bound. Any complete field is necessarily Archimedean,[36] since in any non-Archimedean field there is neither a greatest infinitesimal nor a least positive rational, whence the sequence 1/2, 1/3, 1/4, …, every element of which is greater than every infinitesimal, has no limit.

Since every proper subfield of the reals also contains such gaps, R is the unique complete ordered field, up to isomorphism.[37] Several foundational results in calculus follow directly from this characterization of the reals.

The hyperrealsR* form an ordered field which is not Archimedean. It is an extension of the reals obtained by including infinite and infinitesimal numbers. These are larger, respectively smaller than any real number. The hyperreals form the foundational basis of non-standard analysis.

Another refinement of the notion of a field is a topological field, in which the set F is a topological space, such that all operations of the field (addition, multiplication, the maps a ↦ −a and a ↦ a−1) are continuous maps with respect to the topology of the space.[38] The topology of all the fields discussed below is induced from a metric, i.e., a function

d : F × F → R,

which measures a distance between any two elements of F.

The completion of F is another field in which, informally speaking, the "gaps" in the original field F are filled, if there are any. For example, any irrational numberx, such as x = √2, is a "gap" in the sense that it is a real number that can be approximated arbitrarily closely by rational numbers p/q, in the sense that distance of x and p/q given by the absolute value|x − p/q| is as small as desired. The following table lists some examples of this construction. The fourth column shows an example of a zero sequence, i.e., a sequence whose limit (for n → ∞) is zero.

The field Qp is used in number theory and p-adic analysis. The algebraic closure Qp carries a unique norm extending the one on Qp, but is not complete. The completion of this algebraic closure, however, is algebraically closed. Because of its rough analogy to the complex numbers, it is called the field of complex p-adic numbers and is denoted by Cp.[39]

finite extensions of Fp((t)), the field of Laurent series over Fp (local fields of characteristic p).

These two types of local fields share some fundamental similarities. In this relation, the elements p ∈ Qp and t ∈ Fp((t)) (referred to as uniformizer) correspond to each other. The first manifestation of this is at an elementary level: the elements of both fields can be expressed as power series in the uniformizer, with coefficients in Fp. (However, since the addition in Qp is done using carrying, which is not the case in Fp((t)), these fields are not isomorphic.) The following facts show that this superficial similarity goes much deeper:

Any first order statement which is true for almost all Qp is also true for almost all Fp((t)). An application of this is the Ax-Kochen theorem describing zeros of homogeneous polynomials in Qp.

Adjoining arbitrary p-power roots of p (in Qp), respectively of t (in Fp((t))), yields (infinite) extensions of these fields known as perfectoid fields. Strikingly, the Galois groups of these two fields are isomorphic, which is the first glimpse of a remarkable parallel between these two fields:[41]

where f is an irreducible polynomial (as above).[43] For such an extension, being normal and separable means that all zeros of f are contained in F and that f has only simple zeros. The latter condition is always satisfied if E has characteristic 0.

For a finite Galois extension, the Galois groupGal(F/E) is the group of field automorphisms of F that are trivial on E (i.e., the bijectionsσ : F → F that preserve addition and multiplication and that send elements of E to themselves). The importance of this group stems from the fundamental theorem of Galois theory which constructs an explicit one-to-one correspondence between the set of subgroups of Gal(F/E) and the set of intermediate extensions of the extension F/E.[44] By means of this correspondence, group-theoretic properties translate into facts about fields. For example, if the Galois group of a Galois extension as above is not solvable (can not be built from abelian groups), then the zeros of f can not be expressed in terms of addition, multiplication, and radicals, i.e., expressions involving n{\displaystyle {\sqrt[{n}]{\ }}}. For example, the symmetric groupsSn is not solvable for n≥5. Consequently, as can be shown, the zeros of the following polynomials are not expressible by sums, products, and radicals. For the latter polynomial, this fact is known as the Abel–Ruffini theorem:

Basic invariants of a field F include the characteristic and the transcendence degree of F over its prime field. The latter is defined as the maximal number of elements in F which are algebraically independent over the prime field. Two algebraically closed fields E and F are isomorphic precisely if these two data agree.[47] This implies that any two uncountable algebraically closed fields of the same cardinality and the same characteristic are isomorphic. For example, Qp, Cp and C are isomorphic (but not isomorphic as topological fields).

In model theory, a branch of mathematical logic, two fields E and F are called elementarily equivalent if every mathematical statement which is true for E is also true for F and conversely. The mathematical statements in question are required to be first-order sentences (involving 0, 1, the addition and multiplication). A typical example is

φ(E) = "for any n > 0, any polynomial of degree n in E has a zero in E" (which amounts to saying that E is algebraically closed).

The Lefschetz principle states that C is elementarily equivalent to any algebraically closed field F of characteristic zero. Moreover, any fixed statement φ holds in C if and only if it holds in any algebraically closed field of sufficiently high characteristic.[48]

If U is an ultrafilter on a set I, and Fi is a field for every i in I, the ultraproduct of the Fi with respect to U is a field.[49] It is denoted by

ulimi→∞Fi,

since it behaves in several ways as a limit of the fields Fi: Łoś's theorem states that any first order statement which holds for all but finitely many Fi, also holds for the ultraproduct. Applied to the above sentence φ, this shows that there is an isomorphism[nb 4]

The Ax–Kochen theorem mentioned above also follows from this and an isomorphism of the ultraproducts (in both cases over all primes p)

ulimpQp ≅ ulimpFp((t)).

In addition, model theory also studies the logical properties of various other types of fields, such as real closed fields or exponential fields (which are equipped with an exponential function exp : F → Fx).[50]

For fields which are not algebraically closed (or not separably closed), the absolute Galois groupGal(F) is fundamentally important: extending the case of finite Galois extensions outlined above, this group governs all finite separable extensions of F. By elementary means, the group Gal(Fq) can be shown to be the Prüfer group, the profinite completion of Z. This statement subsumes the fact that the only algebraic extensions of Gal(Fq) are the fields Gal(Fqn) for n > 0, and that the Galois groups of these finite extensions are given by

Gal(Fqn / Fq) = Z/nZ.

A description in terms of generators and relations is also known for the Galois groups of p-adic number fields (finite extensions of Qp).[51]

Algebraic K-theory is related to the group of invertible matrices with coefficients the given field. For example, the process of taking the determinant of an invertible matrix leads to an isomorphism K1(F) = F×. Matsumoto's theorem shows that K2(F) agrees with K2M(F). In higher degrees, K-theory diverges from Milnor K-theory and remains hard to compute in general.

has a unique solution x in F, namely x = b/a. This observation, which is an immediate consequence of the definition of a field, is the essential ingredient used to show that any vector space has a basis.[53] Roughly speaking, this allows choosing a coordinate system in any vector space, which is of central importance in linear algebra both from a theoretical point of view, and also for practical applications.

The function field of an algebraic varietyX (a geometric object defined by polynomial equations) consists of ratios of regular functions, i.e., ratios of polynomial functions f and g. The function field of the n-dimensional space over a field k is k(x1, ..., xn), i.e., the field consisting of ratios of polynomials f and g in n indeterminates. The function field of X is the same as the one of any open dense subvariety. In other words, the function field is insensitive to replacing X by a (slightly) smaller subvariety.

The function field captures important geometric information about X such as its dimension, which equals the transcendence degree of k(X).[55] For curves (i.e., the dimension is one), the function field k(X) is very close to X: if X is smooth and proper (the analogue of being compact), X can be reconstructed, up to isomorphism, from k(X).[nb 5] In higher dimension the function field remembers less, but still decisive information about X. The study of function fields and their geometric meaning in higher dimensions is referred to as birational geometry. The minimal model program attempts to identify the simplest (in a certain precise sense) algebraic varieties with a prescribed function field.

Cyclotomic fields are among the most intensely studied number fields. They are of the form Q(ζn), where ζn is a primitive n-th root of unity, i.e., a complex number satisfying ζn = 1 and ζm ≠ 1 for all m < n.[56] For n being a regular prime, Kummer used cyclotomic fields to prove Fermat's last theorem, which asserts the non-existence of rational nonzero solutions to the equation

xn + yn = zn.

Local fields are completions of global fields. Ostrowski's theorem asserts that the only completions of Q, a global field, are the local fields Qp and R. Studying arithmetic questions in global fields may sometimes be done by looking at the corresponding questions locally. This technique is called the local-global principle. For example, the Hasse–Minkowski theorem reduces the problem of finding rational solutions of quadratic equations to solving these equations in R and Qp, whose solutions can easily be described.[57]

Unlike for local fields, the Galois groups of global fields are not known. Inverse Galois theory studies the (unsolved) problem whether any finite group is the Galois group Gal(F/Q) for some number field F.[58]Class field theory describes the abelian extensions, i.e., ones with abelian Galois group, or equivalently the abelianized Galois groups of global fields. A classical statement, the Kronecker–Weber theorem, describes the maximal abelian Qab extension of Q: it is the field

In addition to the additional structure that fields may enjoy, fields admit various other related notions. Since 0 ≠ 1 in any field, any field has at least two elements. Nonetheless, there is a concept of field with one element which is suggested to be a limit of the finite fields Fp, as p tends to 1.[59] In addition to division rings, there are various other weaker algebraic structures related to fields such as quasifields, near-fields and semifields.

There are also proper classes with field structure, which are sometimes called Fields, with a capital F. The surreal numbers form a Field containing the reals, and would be a field except for the fact that they are a proper class, not a set. The nimbers, a concept from game theory form a Field.[60]

The hairy ball theorem states that a ball can not be combed. More formally, there is no tangent vector field on the sphereS2, which is everywhere non-zero.

Dropping one or several axioms in the definition of a field leads to other algebraic structures. As was mentioned above, commutative rings satisfy all axioms of fields, except for multiplicative inverses. Dropping instead the condition that multiplication is commutative leads to the concept of a division ring or skew field.[nb 6] The only division rings which are finite-dimensional R-vector spaces are R itself, C (which is a field), the quaternionsH (in which multiplication is non-commutative), and the octonionsO (in which multiplication is neither commutative nor associative). This fact was proved using methods of algebraic topology in 1958 by Michel Kervaire, Raoul Bott, and John Milnor.[61] The non-existence of an odd-dimensional division algebra is more classical. It can be deduced from the hairy ball theorem illustrated at the right.

^Some authors also consider the fields R and C to be local fields. On the other hand, these two fields, also called Archimedean local fields, share little similarity with the local fields considered here, to a point that Cassels (1986, p. vi) calls them "completely anomalous".

^Both C and ulimpFp are algebraically closed by Łoś's theorem. For the same reason, they both have characteristic zero. Finally, they are both uncountable, so that they are isomorphic.

^More precisely, there is an equivalence of categories between smooth proper algebraic curves over an algebraically closed field F and finite field extensions of F(T).

^Historically, division rings were sometimes referred to as fields, while fields were called commutative fields.